Editing Ground loop (electricity) (section)

==Common ground loops==
A common type of ground loop is due to faulty interconnections between electronic components, such as laboratory or [[recording studio]] equipment, or home component audio, video, and computer systems.  This creates inadvertent closed loops in the ground wiring circuit, which can allow stray 50/60&nbsp;Hz AC current to be induced and flow through the ground conductors of signal cables.<ref name="Vijayaraghavan8.11">
{{cite web
  | last=Vijayaraghavan
  | first=G.
  | author2=Mark Brown
  | author3=Malcolm Barnes
  | date=December 30, 2008
  | title=8.11 Avoidance of earth loop
  | url=https://www.eetimes.com/electrical-noise-and-mitigation-part-3-shielding-and-grounding-cont-and-filtering-harmonics/
  | access-date=March 24, 2014
  | work=Electrical noise and mitigation - Part 3: Shielding and grounding (cont.), and filtering harmonics|publisher=EDN Network, UBM Tech}}</ref><ref name="Whitlock">
{{cite web
  | last=Whitlock | first=Bill 
  | title=Understanding, finding, and eliminating ground loops in audio and video systems 
  | work=Seminar Template 
  | publisher=Jensen Transformers, Inc. 
  | year=2005 
  | url=http://www.jensen-transformers.com/an/generic%20seminar.pdf 
  | access-date=March 24, 2014 
  | url-status=dead
  | archive-url=https://web.archive.org/web/20090824034929/http://www.jensen-transformers.com/an/generic%20seminar.pdf
  | archive-date=August 24, 2009
}}</ref><ref name="Robinson">
{{cite web
  | last = Robinson
  | first = Larry
  | title = About Ground Loops
  | work = MidiMagic
  | publisher = Larry Robinson personal website
  | year = 2012
  | url = http://midimagic.sgc-hosting.com/gndloop.htm
  | access-date = March 24, 2014
}}</ref><ref name="Ballou">
{{cite book
  | last1  = Ballou | first1 = Glen 
  | title  = Handbook for Sound Engineers
  | publisher = Taylor and Francis
  | edition = 4
  | date   = 2008
  | pages  = 1194–1196
  | url    = https://books.google.com/books?id=eh60Ue_K2QkC&q=%22ground+loop&pg=PA1194
  | isbn   = 978-1136122538
}}</ref> The voltage drops in the ground system caused by these currents are added to the signal path, introducing noise and hum into the output. The loops can include the building's utility wiring ground system when more than one component is grounded through the protective earth (third wire) in their power cords.

===Ground currents on signal cables===
[[Image:Signal cable with ground current.svg|thumb|upright=1.5|Fig. 1: A typical signal cable ''S'' between electronic components, with a current ''I'' flowing through the shield conductor]]

The symptoms of a ground loop, ground noise and hum in electrical equipment, are caused by current flowing in the ground or {{em|shield}} conductor of a cable.  Fig. 1 shows a signal cable ''S'' linking two electronic components, including the typical [[line driver]] and receiver amplifiers ''(triangles)''.<ref name="Robinson" />  The cable has a ground or shield conductor which is connected to the chassis ground of each component.   The driver amplifier in component 1 ''(left)'' applies signal ''V''<sub>1</sub> between the signal and ground conductors of the cable.  At the destination end ''(right)'', the signal and ground conductors are connected to a [[differential amplifier]].  This produces the signal input to component 2 by subtracting the shield voltage from the signal voltage to eliminate [[common-mode interference|common-mode noise]] picked up by the cable
<math display="block">V_2 = V_\text{S2} - V_\text{G2} \,</math>

If a current ''I'' from a separate source is flowing through the ground conductor, the resistance ''R'' of the conductor will create a voltage drop along the cable ground of ''IR'', so the destination end of the ground conductor will be at a different potential than the source end 
<math display="block">V_\text{G2} = V_\text{G1} - IR \,</math>
Since the differential amplifier has high impedance, little current flows in the signal wire, therefore there is no voltage drop across it: <math>V_\text{S2} = V_\text{S1} \,</math>  The ground voltage appears to be in series with the signal voltage ''V''<sub>1</sub> and adds to it
<math display="block">V_2 = V_\text{S1} - (V_\text{G1} - IR)\,</math>
<math display="block">V_2 = V_1 + IR\,</math>

If ''I'' is an AC current this can result in noise added to the signal path in component 2.

===Sources of ground current===
The diagrams in this section show a typical ground loop caused by a signal cable ''S'' connecting two grounded electronic components ''C1'' and ''C2''.  The loop consists of the signal cable's ground conductor, which is connected through the components' metal chassis to the ground wires ''P'' in their power cords, which are plugged into outlet grounds which are connected through the building's utility ground wire system ''G''.

Such loops in the ground path can cause currents in signal cable grounds by two main mechanisms:
* [[Image:Ground loop - induced currents.svg|thumb|upright=1.2|Ground loop current induced by stray AC [[magnetic field]]s ''<span style="color:green;">(B, green)</span>'']] Ground loop currents can be induced by stray AC [[magnetic field]]s<ref name="Robinson" /><ref name="Vijayaraghavan8.8.3">
{{cite web
  | last = Vijayaraghavan | first = G.
  | author2=Mark Brown
  | author3=Malcolm Barnes
  | title = 8.8.3 Magnetic or inductive coupling
  | work = Electrical noise and mitigation - Part 3: Shielding and grounding (cont.), and filtering harmonics
  | publisher = EDN Network, UBM Tech
  | date = December 30, 2008
  | url = http://www.eetimes.com/document.asp?doc_id=1274126&page_number=3
  | access-date = March 24, 2014
}}</ref> ''<span style="color:green;">(B, green)</span>'' which are always present around AC electrical wiring.  The ground loop constitutes a conductive wire loop which may have a large area of several square meters.  According to [[Faraday's law of induction]], any time-varying [[magnetic flux]] passing through the loop induces an [[electromotive force]] (EMF) in the loop, causing a time varying current to flow.  The loop acts like a [[short circuit]]ed single-turn transformer winding; any AC [[magnetic flux]] from nearby transformers, electric motors, or just adjacent power wiring, will induce AC currents in the loop by induction.  In general, the larger the area spanned by the loop and the larger the magnetic flux through it, the larger the induced currents will be.   Since its [[Electrical resistance|resistance]] is typically very low, often less than 1 [[ohm]], the induced currents can be large.{{clear}}
* [[Image:Ground loop - leakage currents.svg|thumb|upright=1.6|Ground loop current caused by leakage currents in the building's ground wire system from an appliance ''A'']] Another less common source of ground loop currents, particularly in high-power equipment, is current leaking from the ''hot'' side of the power line into the ground system.<ref name="Vijayaraghavan8.11" /><ref name="Ballou2">This type is often called "common impedance coupling", [https://books.google.com/books?id=eh60Ue_K2QkC&pg=PA1198&dq=%22common+impedance+coupling   Ballou 2008 ''Handbook for Sound Engineers, 4th Ed.'', p. 1198-1200]</ref> In addition to resistive leakage, current can also be induced through low impedance capacitive or inductive coupling. The ground potential at different outlets may differ by as much as 10 to 20 volts<ref name="Whitlock" /> due to voltage drops from these currents. The diagram shows leakage current from an appliance such as an electric motor ''A'' flowing through the building's ground system ''G'' to the [[neutral wire]] at the utility ground bonding point at the [[electrical service panel|service panel]]. The ground loop between components ''C1'' and ''C2'' creates a second parallel path for the current.<ref name="Ballou2" /> The current divides, with some passing through component ''C1'', the signal cable ''S'' ground conductor, ''C2'' and back through the outlet into the ground system ''G''. The AC voltage drop across the cable's ground conductor from this current introduces hum or interference into component ''C2''.<ref name="Ballou2" />{{clear}}

===Solutions===
The solution to ground loop noise is to break the ground loop, or otherwise prevent the current from flowing. Several approaches are available.
* Group the cables involved in the ground loop into a bundle or [[Snake cable|snake]].<ref name="Vijayaraghavan8.11" />  The ground loop still exists, but the two sides of the loop are close together, so stray magnetic fields induce equal currents in both sides, which cancel out.
* [[Image:Ground loop solution - broken shield.svg|thumb|Break in the shield]] Create a break in the signal cable shield conductor.<ref name="Robinson" />  The break should be at the load end.  This is often called ''ground lifting''.  It is the simplest solution; it leaves the ground currents to flow through the other arm of the loop.  Some sound system components have ground lifting switches at inputs, which disconnect the ground.  One problem with this solution is if the other ground path to the component is removed, it will leave the component ungrounded and stray leakage currents may cause a very loud hum in the output, possibly damaging speakers.{{clear}}
* [[Image:Ground loop solution - resistor.svg|thumb|Resistor in the shield]] Put a small resistor of about 10{{nbsp}}Ω in the cable shield conductor, at the load end.<ref name="Robinson" />  This is large enough to reduce magnetic-field-induced currents but small enough to keep the component grounded if the other ground path is removed. In high-frequency systems this solution leads to impedance mismatch and leakage of the signal onto the shield, where it can radiate to create [[radio frequency interference|RFI]], or, symmetrically through the same mechanism, external signals or noise can be received by the shield and mixed into the desired signal. {{clear}}
* [[Image:Ground loop solution - isolation transformer.svg|thumb|Isolation transformer]] Use a ground loop [[isolation transformer]] in the cable.<ref name="Whitlock" /><ref name="Robinson" />  This is considered the best solution, as it breaks the DC connection between components while passing the [[differential signal]] on the line.  Even if one or both components are ungrounded, no noise will be introduced.  The better isolation transformers have grounded shields between the two sets of windings.  A transformer generally introduces some distortion in [[frequency response]].  A transformer designed specifically for the relevant frequency range must be used. [[Optoisolator]]s can perform the same task for digital lines but introduce signal delay.{{clear}}
* In circuits producing high-frequency noise such as computer components, [[ferrite bead]] [[Choke (electronics)|chokes]] are placed around cables just before the termination to the next appliance (e.g., the computer). These present a high impedance only at high frequency, so they will effectively stop radio frequency and digital noise, but will have little effect on 50/60&nbsp;Hz noise.
* Reinforce the shield of the signal cable connecting C1 and C2 by connecting a thick copper conductor in parallel to the shield. This reduces the resistance of the shield and thus the amplitude of the unwanted signal.
* A technique used in recording studios is to interconnect all the metal chassis with heavy conductors like copper strips, then connect to the building ground wire system at ''one'' point; this is referred to as ''star grounding'' or ''single-point grounding''.  However, in home systems, multiple components are usually grounded through their 3-wire power cords, resulting in multipoint grounds.
* [[Electric battery|Battery]]-powering one or more of the circuits can avoid a ground loop, because the entire device may be disconnected from mains power.

A hazardous technique sometimes used by amateurs is to break the ''third wire'' ground conductor ''P'' in one of the component's power cords, by removing the ground pin on the plug, or using a [[cheater plug]].  This creates an electric shock hazard by leaving one of the components ungrounded.<ref name="Whitlock" /><ref name="Robinson" />

====Balanced lines====
A more comprehensive solution is to use equipment that employs [[differential signaling]].  Ground noise can only get into the signal path in [[single-ended signaling]], in which the ground or shield conductor serves as one side of the signal path.  When the signal is sent as a differential signal along a pair of wires, neither of which are connected to ground, any noise from the ground system induced in the signal lines is a [[common-mode signal]], identical in both wires.  Since the line receiver at the destination end only responds to differential signals, a difference in voltage between the two lines, the common-mode noise is canceled out.  Thus these systems are very immune to electrical noise, including ground noise.  Professional and scientific equipment often uses differential signaling with [[balanced line]]s.